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CVE-2026-81725— NLTK before 3.10.3 Regular Expression Denial of Service via Pl196xCorpusReader

Quick assessment

Affected
nltk nltk
Exploitation
No confirmed in-the-wild exploitation; assess based on exposure
Recommended action
Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.

NLTK 3.10.3 之前的版本在 中存在正则表达式拒绝服务（ReDoS）漏洞。攻击者可以通过提供包含大量未匹配开标签的畸形 TEI 块，导致二次方级的 CPU 消耗。攻击者可以利用 方法中的惰性正则表达式模式，通过 和 等公共 API 强制进行重复的重新扫描，从而实现近二次方级的运行时增长。

CVSS 3.7 · Low

Possible ATT&CK Techniques 1 AI

T1499 · Endpoint Denial of Service

Affected Version Matrix 2

VendorProduct Version RangeStatus
nltk nltk < 3.10.3 affected
3.10.3 unaffected
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I. Basic Information for CVE-2026-81725

Vulnerability Information
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Vulnerability Title
NLTK before 3.10.3 Regular Expression Denial of Service via Pl196xCorpusReader
Source: CVE Program / CVE List V5
Vulnerability Description
NLTK before 3.10.3 contains a regular expression denial of service vulnerability in Pl196xCorpusReader that allows attackers to cause quadratic CPU consumption by supplying malformed TEI blocks with many unmatched opening tags. Attackers can exploit lazy regex patterns in the read_block method through public APIs like words() and tagged_words() to force repeated rescans and achieve near-quadratic runtime growth.
Source: CVE Program / CVE List V5
CVSS Information
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L
Source: CVE Program / CVE List V5
Vulnerability Type
未加控制的资源消耗(资源穷尽)
Source: CVE Program / CVE List V5

Affected Products

Vendor Product Affected Versions CPE Subscribe
nltk nltk 0 ~ 3.10.3 -

II. Public POCs for CVE-2026-81725

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III. Intelligence Information for CVE-2026-81725

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Vendor Advisories for CVE-2026-81725 (2)

Same Patch Batch · nltk · 2026-08-27 · 6 CVEs total

CVE-2026-81722 7.5 HIGH nltk PorterStemmer before 3.10.3 Quadratic-time DoS
CVE-2026-81727 7.1 HIGH NLTK before 3.10.3 Hardlink File Overwrite via downloader
CVE-2026-81726 7.0 HIGH NLTK through 3.10.3 Path Traversal via Model-Artifact APIs
CVE-2026-81724 5.3 MEDIUM NLTK before 3.10.3 Denial of Service via Uncontrolled Recursion
CVE-2026-81723 3.7 LOW NLTK before 3.10.3 Quadratic CPU Exhaustion via XMLCorpusView

IV. Related Vulnerabilities

Same product: nltk
Same vendor: nltk
Same weakness: CWE-400

V. Comments for CVE-2026-81725

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